This invention relates to non-destructive testing methods and apparati for determining the xe2x80x9ccleanliness,xe2x80x9d that is, degree of material internal purity, of metallic sputter target materials and, more particularly, non-destructive methods and apparati for determining cleanliness based on the sound propagation properties of the materials.
Cathodic sputtering is widely used for depositing thin layers or films of materials from sputter targets onto desired substrates such as semiconductor wafers. Basically, a cathode assembly including a sputter target is placed together with an anode in a chamber filled with an inert gas, preferably argon. The desired substrate is positioned in the chamber near the anode with a receiving surface oriented normally to a path between the cathode assembly and the anode. A high voltage electric field is applied across the cathode assembly and the anode.
Electrons ejected from the cathode assembly ionize the inert gas. The electrical field then propels positively charged ions of the inert gas against a sputtering surface of the sputter target. Material dislodged from the sputter target by the ion bombardment traverses the chamber and deposits on the receiving surface of the substrate to form the thin layer or film.
One factor affecting the quality of the layer or film produced by a sputtering process is the xe2x80x9ccleanlinessxe2x80x9d of the material from which the sputter target is made. The term xe2x80x9ccleanlinessxe2x80x9d is widely used in the semiconductor industry, among others, to characterize high purity and ultra high purity materials. In common practice, xe2x80x9ccleanlinessxe2x80x9d refers to the degree of material internal purity. Such impurities may be present, for example, as inclusions of impurity-rich phases surrounded by the sputter target material. Cleanliness is usually measured in units of particles per million (xe2x80x9cppmxe2x80x9d) or particles per billion (xe2x80x9cppbxe2x80x9d) which define a ratio between the number of contaminant atoms and the total number of atoms sampled.
Since the cleanliness of the material from which a sputter target is made affects the quality of layers of films produced using that target, it is obviously desirable to use relatively clean materials in fabricating sputter targets. This implies a need in the art for non-destructive techniques for selecting sputter target blanks of suitable cleanliness to produce high quality sputter targets. Known destructive test methods, such as glow discharge mass spectroscopy and LECO techniques, are not suitable for this purpose.
Another factor affecting the quality of the layer or film produced by a sputtering process is the presence of xe2x80x9cflawsxe2x80x9d in the sputter target material. As used herein, the term xe2x80x9cflawsxe2x80x9d refers to microscopic volumetric defects in the sputter target material, such as inclusions, pores, cavities and micro-laminations. Since flaws in a sputter target affect the quality of layers or films produced using that target, there exists a corresponding need in the art for non-destructive techniques for characterizing flaws present in sputter target materials.
FIG. 1 illustrates a prior art non-destructive ultrasonic xe2x80x9cflawxe2x80x9d detection method for characterizing aluminum and aluminum alloy sputter target materials. The technique illustrated in FIG. 1 is similar to that suggested in Aluminium Pechiney PCT Application No. PCT/FR96/01959 for use in classifying aluminum or aluminum alloy blanks suitable for fabricating sputter targets based on the size and number of internal xe2x80x9cdecohesionsxe2x80x9d detected per unit volume of the blanks.
The prior art technique of FIG. 1 employed a pulse-echo method performed on a test sample 10 having a planar upper surface 12 and a parallel planar lower surface 14. In accordance with this technique, focused ultrasonic transducer 16 irradiated a sequence of positions on the upper surface 12 of the test sample 10 with a single, short-duration, high-frequency ultrasound pulse 18 having a frequency of at least 5 MHz, and preferably 10-50 MHz. The ultrasonic transducer 16 then switched to a sensing mode and detected a series of echoes 20 induced by the ultrasound pulse 18.
One factor contributing to these echoes 20 was scattering of sonic energy from the ultrasound pulse 18 by flaws 22 in the test sample 10. By comparing the amplitudes of echoes induced in the test sample 10 with the amplitudes of echoes induced in reference samples (not shown) having compositions similar to that of the test sample 10 and blind, flat-bottomed holes of fixed depth and diameter, it was possible to detect and count flaws 22 in the test sample 10.
The number of flaws detected by the technique of FIG. 1 had to be normalized in order to facilitate comparison between test samples of different size and geometry. Conventionally, the number of flaws was normalized by volumexe2x80x94that is, the sputter target materials were characterized in units of xe2x80x9cflaws per cubic centimeter.xe2x80x9d The volume associated with the echoes 20 from each irradiation of the test sample 10 was determined, in part, by estimating an effective cross-section of the pulse 18 in the test sample 10.
One drawback to the technique of FIG. 1 is that a number of factors detract from the ability of the transducer 16 to detect sonic energy scattered by the flaws 22. This reduces the sensitivity of the technique.
One such factor is relative weakness of the scattered energy. A portion of the scattered energy is attenuated by the material making up the test sample 10. Furthermore, since the flaw sizes of interest, which range from approximately 0.04 mm to 0.1 mm, are significantly less than the wavelength of ultrasound in metals (for example, the wavelength of sound in aluminum for the frequency range of 10 MHz to 50 MHz is 0.6 mm to 0.12 mm, respectively), the pulse 18 has a tendency to refract around the flaws 22, which reduces the scattering intensity.
Another factor detracting from the ability of the transducer 16 to detect the sonic energy scattered by the flaws 22 is the noise generated by scattering of the pulse 18 at the boundaries between grains having different textures. In fact, the texture-related noise can be so great for high-purity aluminum having grain sizes on the order of several millimeters that small flaws within a size range of approximately 0.05 mm and less cannot be detected. Larger grain sizes reduce the signal-to-noise ratio for the sonic energy scattered by the flaws when compared to the noise induced by the grain boundaries.
Other factors affecting the sensitivity and resolution of the technique of FIG. 1 includes the pulse frequency, duration and waveform; the degree of beam focus and the focal spot size; the coupling conditions (that is, the efficiency with which the sonic energy travels from the transducer 16 to the test sample 10); and the data acquisition system parameters.
Another drawback to the technique of FIG. 1 is that the calculation of the xe2x80x9cflaws per cubic centimeterxe2x80x9d in the test sample 10 presupposes that only flaws 22 within a determinable cross-sectional area scatter sonic energy back toward the transducer 16. In fact, the pulse 18, due to its wave nature, does not have localized, well-determined boundaries.
The distribution of the energy of the pulse 18 within the test sample 10, under simplifying assumptions, permits one to define a corridor 30 having a determinable cross-section beneath the transducer 16 in which most of the energy should be concentrated. Nevertheless, some of the energy of the pulse 18 will propagate outside this corridor 30. As a result, the transducer may detect sonic energy scattered by relatively large flaws 22 located outside the estimated corridor 30, thereby overestimating the density of flaws 22 in the test sample 10 and underestimating their sizes. Therefore, material cleanliness characteristics become to some degree uncertain.
Thus, there remains a need in the art for non-destructive techniques for characterizing sputter target materials having greater sensitivity than methods in the prior art. There also remains a need for techniques which permit the comparison of the cleanliness of different sputter target materials in a manner which is not dependent on arbitrary volumetric estimations in the form xe2x80x9cflow per cubic unit.xe2x80x9d One conventional imaging technique for sputter target material is C-scanning. It maps the flaws on a two-dimensional image of the material sample. Where the size of the tested object is on the order of approximately ten centimeters or greater, however, it becomes difficult to indicate the relative sizes of flaws having diameters on the order of approximately 0.04 mm to 0.1 mm to any realistic scale. When computerized imaging is used it may be impossible to indicate the relative sizes of flaws in this manner where the sizes of the flaws relative to the entire width or diameter of the sample surface are less than the relative pixel sizes of the display device.
Therefore there remains an additional need in the art for an imaging technique which does not require the display of flaws scaled relative to the surface area of the test sample.
These needs and others are addressed by a non-destructive method for characterizing a sputter target material comprising the steps of sequentially irradiating a test sample of the sputter target material with sonic energy at a plurality of positions on a surface of the sample; detecting echoes induced by the sonic energy; discriminating texture-related backscattering noise from the echoes to obtain modified amplitude signals; comparing the modified amplitude signals with said at least one calibration value to detect flaw data points and no-flaw data points (that is, data points in which flaws were detected, and were not detected, respectively); counting the flaw data points to determine a flaw count CF; counting the flaw data points and the no-flaw data points to determine a total number of data points CDP; and calculating a cleanliness factor FC=(CF/CDP)xc3x97106.
Unlike the prior art method described earlier, the method of the present invention provides a characterization of the sputter target material which is not dependent on theoretical estimates of the cross-sectional area occupied by the sonic energy during its flight through the test sample. Since the cleanliness factor is normalized by the number of data points rather than by estimated volume, there is less risk of overestimating the number of flaws, or underestimating their sizes, than in the prior art method of FIG. 1.
Although the cleanliness factor provides a useful characterization of the sputter target material, more information can be provided by means of a histogram. More specifically, the sputter target may be characterized by defining a plurality of amplitude bands; measuring said modified amplitude signals to determine modified amplitude signal magnitudes; comparing said modified amplitude signal magnitudes with said plurality of amplitude bands to form subsets of said modified amplitude signals; counting said subsets of modified amplitude signals to determine a plurality of modified amplitude signal counts, each modified amplitude signal count of said plurality of amplitude signal counts corresponding to one of said amplitude bands of said plurality of amplitude bands; and constructing a histogram relating said modified signals counts to said plurality of amplitude bands. Since the histogram does not attempt to directly map the locations of flaws along the surface of the sputter target material, it does not suffer from the scaling problems inherent in prior art mapping techniques.
Most preferably, the test sample is compressed along one dimension, such as by rolling or forging, and then irradiated by sonic energy propagating transversely (that is, obliquely or, better yet, normally) to that dimension. This has the effect of flattening and widening any flaws in the material. The widening of the flaws, in turn, increases the intensity of the sonic energy scattered by the flaws and reduces the likelihood that the sonic energy will refract around the flaws.
These methods for characterizing sputter target materials may be used in processes for manufacturing sputter targets. As noted earlier, the cleanliness of a sputter target is one factor determining the quality of the layers or films produced by the target. By shaping only those sputter target blanks having cleanliness factors or histograms meeting certain reference criteria to form sputter targets, and rejecting blanks not meeting those criteria, one improves the likelihood that the sputter targets so manufactured will produce high quality layers or films.
Therefore, it is one object of the invention to provide non-destructive methods for characterizing sputter target materials. Other objects of the invention will be apparent from the follow description the accompanying drawings, and the appended claims.